The present invention relates to a rubber composition improving the reaction coefficient of a silane coupling agent and various physical properties.
Recently, compatibility between lowering the fuel cost of a tire (reduction in rolling resistance) and improving wet grip performance (brake performance) has been intensely required and silica is compounded in a tread in order to satisfy the requirement. However, in general, silica is hardly dispersed in a rubber composition and sufficient reinforcing property for the rubber composition can not be obtained only by silica; therefore, a silane coupling agent is used at the same time in combination with silica. Further, a silane coupling agent containing an alkoxyl group is generally used for tire use. However, the reaction of the silane coupling agent cannot be sufficiently completed in the step of kneading a rubber and the silane coupling agent cannot but be compounded beyond necessity in order to obtain sufficient reinforcing property. Further, there has been a problem that a silane coupling agent (hereinafter, referred to as an unreacted silane coupling agent) which cannot be reacted during kneading causes a reaction in an extrusion step to generate alcohol (such as ethanol) and air bubbles are generated in an unvulcanized tread.
As a method of solving the problem, JP-A-2001-247718 discloses that various sodium salts are compounded with a rubber composition. Although the reaction efficiency of a silane coupling agent can be improved by the method, effect of suppressing generation of air bubbles in an extruded tread is not sufficiently obtained by sodium salts and the generation of air bubbles in the extruded tread could not be perfectly suppressed. Further, there has been a problem that a sodium salt is not sufficiently dispersed in a rubber composition.